The present invention relates to chopper-stabilized power supplies and more particularly to chopper devices and regulating devices for stabilized supplies capable of supplying high current intensities. These supplies are adapted to furnish a high and variable direct current at constant voltage with maximum efficiency from a variable and unstabilized source of d-c energy.
The underlying principle of chopper-stabilized supplies is the following: a switch, usually a transistor, chops the current furnished by the unregulated source of energy at a rate set by a pilot circuit. The chopped current is applied to an integrating filter constituted by an induction coil in series with the transistor and a capacitor connected to the terminals of the circuit of utilization. The output voltage is compared with a reference voltage by a pilot circuit which switches the switching transistor off or on. As soon as the switch is closed, an increasing current flows in the induction coil and in the capacitor. When the switch opens, the energy stored in the induction coil is restored in the form of a current which flows in the induction coil and decreases owing to a recovery diode which closes the induction coil onto the output. The pilot circuit which controls the opening and closing of the switch acts as a voltage-duration or voltage-frequency converter. It converts the difference between the output voltage and a reference voltage into pulses of variable width or variable repetition frequency and applies them through a power amplifier to the switching transistor.
In order to ensure that the transistor performs its switching function without excessive losses, it is essential that it be saturated in respect of any intensity of the current liable to pass therethrough. When this condition is satisfied, the main energy losses are due to the transitions of transistor saturation and cut-off.
The saturation of the transistor requires the injection of a base current and poses no particular problem. Indeed, it is known to apply to the transistor a base current which increases as the collector current by inserting an induction in the base circuit. The base current for saturating the transistor is therefore low and during the closure of the transistor, the control current increases as the supplied current.
But during the cut-off stage the charge carriers which remain in the base of the transistor prevent the sudden stoppage of the collector current and a non-negligible proportion of the energy is dissipated in the transistor.
An object of the invention is to reduce the duration of the cut-off stage of the switching transistor.